Under stairs toilet

We currently have a toilet in the under stairs cupboard but it's such a small space we can't close the door when on the toilet! We have another half cupboard next to it. Would it be feasible to knock through to create a bigger area and also install a small sink? There are (I think boiler) pipes running through the 2 cupboards, and a gas metre in the half cupboard, as well as the slope of the stairs to contend with. But keen to explore this possibility to make into a more functionable toilet.